This invention generally relates to the field of communication systems and, more particularly, to the improvement of receiver sensitivity, adjacent-channel and co-channel rejection in a communication system that sub-divides RF channels into time slots.
Time division multiple access (TDMA) communication systems, such as those based on GSM, PDC, D-AMPS communication standards, subdivide one or more radio frequency (RF) channels into a plurality of time slots during which mobile stations within the system engage in voice and data communication. By transmitting and receiving bursts of information during allocated time slots, the mobile stations communicate through a plurality of scattered base stations, which cover corresponding communication cells. Within each cell, a base station communicates with the mobile stations over a number of RF channels by transmitting bursts of downlink data to the mobile stations over downlink RF channels and receiving bursts of uplink data from the mobile stations over uplink RF channels.
To avoid co-channel interference, neighboring cells are allocated different RF channels. Because of the relatively low power RF transmissions within a particular cell, another cell spaced two or more cells apart may typically reuse the same frequency. The farther the cells reusing the same frequencies are from each other, the lower the co-channel interference between them. Therefore, lowering co-channel interference of the uplink channels increases system capacity by allowing a tighter frequency reuse cell pattern in a dense communication network.
It is also very important to provide wide coverage using fewer number of base stations, to reduce implementation cost of the network. One factor that is directly related to a base station's coverage is its sensitivity to received signals on the uplink channel. Some conventional systems equip the base stations with low noise amplifiers, which are positioned at close proximity to base station antennas.
Another conventional method to improve both co-channel interference and receiver sensitivity uses receiver diversity under which two or more spaced apart antennas that are coupled to corresponding receiver branches receive uplink signals from the mobile station. Based on well known principles that relate to combining non-coherent noise and desired signals, received signal quality is improved by combining received signals from the receivers. For example, some conventional GSM systems combine uplink signals received by two receiver branches at the base station, to improve uplink received signal quality on a continuous basis. Other systems combine received signals from two or more receiver branches, when the received signal strength from a mobile station at a single receiver drops below a predefined threshold.
Conventional methods, however, do not improve co-channel interference and receiver sensitivity on a time-slot by time-slot basis. Because TDMA systems sub-divide RF channels into time slots, the received signal quality may be degraded during a time slot of the uplink channel that is assigned to a particular mobile station. For example, the receiver sensity of a signal received from a mobile station at the fringes of a communication cell may be well below a desired level, requiring improvement during its allocated time-slot. Although conventional TDMA systems use receiver diversity continuously when receiving uplink RF signals, they do not provide for receiver diversity on a time-slot by time-slot basis. Therefore, there exists a need to improve received signal quality of an uplink RF channel that is subdivided into time slots during a selected time slot.